Wastewater treatment using electrolysis with activated carbon cathode

ABSTRACT

AN IMPROVEMENT IN THE ELECTROLYTIC TREATMENT OF WASTEWATER IS DESCRIBED IN WHICH THE REDUCTION OF OXYGEN ON AN ACTIVATED CARBON CATHODE IS EMPLOYED TO FORM HYDROGEN PEROXIDE, WHICH IN TURN SERVES TO OXIDIZE ORGANIC CARBON POLLUTANTS IN THE WASTEWATER. THE OXYGEN IS FED TO THE ACTIVATED CARBON CATHODE FROM OUTSIDE THE ELECTROLYTIC   CEIL, WITH THE CATHODE MEMBER DEFINING AT LEAST PART OF THE ENCLOSURE FOR FEEDING THE GAS TO THE ACTIVATED CARBON CATHODE SURFACE, AND WITH THE CATHODE SURFACE FORMING PART OF THE BOUNDRY OF THE CELL COMPARTMENT.

Jan. 29, 1974 MASAYUKI KAWAHATA ETA!- WASTEWATER TREATMENT USINGELECTROLYSIS WITH ACTIVATED CARBON CATHODE Original Filed Feb. 5. 1969United States Patent Office Patented Jan. 29, 1974 3,788,967 WASTEWATERTREATMENT USING ELECTROL- YSIS WITH ACTIVATED CARBON CATHODE MasayukiKawahata, Scotia, and Kenneth R. Price,

Schenectady, N.Y., assignors to General Electric Com- P y Originalapplication Feb. 3, 1969, Ser. No. 795,917. Divided and this applicationOct. 20, 1971, Ser. No. 191,103

Int. Cl. B01k 3/08 US. Cl. 204-277 3 Claims ABSTRACT OF THE DISCLOSUREAn improvement in the electrolytic treatment of wastewater is describedin which the reduction of oxygen on an activated carbon cathode isemployed to form hydrogen peroxide, which in turn serves to oxidizeorganic carbon pollutants in the wastewater. The oxygen is fed to theactivated carbon cathode from outside the electrolytic cell, with thecathode member defining at least part of the enclosure for feeding thegas to the activated carbon cathode surface, and with the cathodesurface forming part of the boundary of the cell compartment.

BACKGROUND OF THE INVENTION This is a division of application Ser. No.795,917, filed Feb. 3, 1969 and assigned to the assignee of the instantapplication.

The use of an activated carbon cathode for the commercial production ofhydrogen peroxide by oxygen reduction in an electrolytic cell has beendescribed in US. Pat. 2,000,815Berl and, also, the depolarization of thecathode in an air cell battery by supplying air (or oxygen) underpressure to a cathode having a surface of activated carbon is describedin US. Pat. 2,275,281-Berl; further, the application of electrolysis towastewater treatment, including electrocoagulation, has beeninvestigated in the past.

However, particularly in those cases in which waste- Waters ofrelatively low conductivity require processing, electrochemicaltreatment has not been economical. The application of electrochemicaltreatment to wastewater processing is, therefore, in need of animprovement in which the energy consumption of the process is reduced.

SUMMARY OF THE INVENTION A treatment system and method for achievinghigher efficiencies are made available by the instant invention whereinuse is made of the cathodic reduction of oxygen to form hydrogenperoxide which reacts with organic contaminants in wastewater. A hollowporous activated carbon cathode is the means by which oxygen-containingflow is introduced into a system wherein wastewater passes between thecathode surface and an anode surface spaced therefrom. When current ispassed through the wastewater, oxygen is reduced at the cathode and isconverted to hydrogen peroxide in the presence of the activated carbon,which then becomes available for more effective oxidation of organicpollutants in the wastewater than would be accomplished by straightelectrolysis.

BRIEF DESCRIPTION OF THE DRAWING The exact nature of this invention aswell as objects and advantages thereof will be readily apparent fromconsideration of the following specification relating to the annexeddrawing in which:

FIG. 1 is an elevational view partly cut away to show the means forintroducing wastewater to, and discharging wastewater from, the spacebetween the anode and the cathode constructions and FIG. 2 is anenlarged view in section and partially cut away of a length of treatmentunit such as may exist between lines A-A and BB in the structure of FIG.1.

DESCRIPTION OF THE PREFERRED EMBODIMENT In the exemplary treatmentdevice 10 shown in FIG. 1, wastewater to be treated enters manifold 11via inlet 12 being admitted to the annular space between anode 13 andactivated carbon cathode 14 via holes 16 spaced around the circumferenceof anode 13. Oxygen or an oxygen-containing gas, such as air, underpressure is supplied from a source (not shown) to the interior of hollowcathode 14 (closed off at the lower end thereof to force passage of theoxygen through the cathode structure) for passage radially outwardthrough the porous activated charcoal wall. The rate of admission of theoxygenating gas should be more than sufiicient to saturate thewastewater fiow with oxygen. During the passage of wastewater throughthe unit 10, electrical potential is applied across electrodes 13 and 14from a direct current power source, such as battery 17.

Under the application of the aforementioned electrical potential,current passes through the wastewater disposed between electrodes 13 and14 and electrolysis of the water occurs to liberate hydrogen at cathode14, which hydrogen reacts over the contact surface of the activatedcarbon with oxygen passing out through the cathode wall 14 to formhydrogen peroxide. The hydrogen peroxide made available in this mannereffectively oxidizes the organic pollutants (suspended or dissolved)present both in the wastewater and adsorbed on the outer surface ofcathode 14. At the same time, when ferrous metal or aluminum areemployed as the anode surface fluocculant is gener ated (by dissociationof the anode and reaction with water) and such flocculant material willhelp to remove suspended solids, as would be present in raw wastewaterflow. Ferrous metal anodes are preferred, however, because the oxidationof the organic carbon by the hydrogen peroxide is enhanced in thepresence of ferrous ions. Lead oxide may also be used for the anode.

Important advantages obtained by the use of the oxygenating gas input tothe wastewater treatment system are as follows:

(a) The combined presence of Fe++ and H 0 in a system saturated withoxygen leads to reactions in which the Fe++ and the oxygen reactpreferentially to form Fe(OH) xH O (flocculant) leaving the H 0undepleted and available for reaction with the organic carbon insolution. In the absence of significant amounts of oxygen in thewastewater the effectiveness of oxidation of oxidation of the organiccarbon by H 0 generated in the system would be sharply reduced, becauseof the preferential reaction, which occurs between Fe++ and the H 0 bywhich the H O is greatly diminished,

(b) The formation of ferric floc masses (floccules) in preference toferrous floccules has the benefit of increased suspended particleremoval, because ferric floccules are larger and have greater surfacearea with more electric charges available for the attraction ofsuspended particles, and

(c) The vigorous bubbling of oxygen through the wastewater flow producesan effiuent from the system saturated with oxygen (between about 7 to 11mg. 0 liter depending on the temperature). Having the wastewate r sosaturated with oxygen is of considerable benefit to recipient streams.

The treated wastewater exits from device 10 via holes 21, manifold 22and outlet pipe 23. In FIG. 2 a more detailed arrangement of a suitableelectrode construction is shown.'Anode'13 and cathode'structure 14 areheld in spaced concentric relation by spacers 24, cathode structure 14being composed of concentric perforated metal cylinder 26 and screen 27(which preferably is not electrically conducting) with the activatedcarbon granules 28 therebetween.

The construction shown is exemplary as the oxygencontaining gas conduitand adjacent anodes could have other shapes. For example, the cathodeconduit could be of rectangular cross-section defining open-top troughsto either side thereof together with vertical longitudinallyextendinganodes spaced (and electrically insulated) therefrom. Also, if desired,the activated carbon material could be disposed at the cathode as acoating on a porous electrically-conducting substrate. The arrangementshown is suitable for a continuous process, although a batch operationmay be employed.

Activated carbon employed should have a surface of at least 50 sq.meters/gram and may, for example, be prepared by treating oil refineryacid sludge with a neutralizing agent and heating the mixture to 800 to1200 C. as described in U.S. Pat. 1,812,316Berl. The most effectivecarbon electrodes are those which have a large porous surface area,these being most active in catalyzing both the formation and sequentialdecomposition of hydrogen peroxide.

Organic carbon occurs in untreated wastewater both in the form ofsuspended solids and as dissolved solids. Removal of the organic carbonpresent as suspended solids and as dissolved solids. Removal of theorganic carbon present as suspended solids in such wastewater may beeffectively accomplished in several ways as, for example, by theaddition thereto of various organic and inorganic floccnlant materialsor by the use of electrocoagulation (by which flocculant material isgeneral in situ). The removal of organic carbon in the form of dissolvedsolids presents a much more diflicult problem, however. For example,tests have shown that electrocoagulation by itself is not efiective tosignificantly reduce dissolved solid content. Further, although there issome indication (Electrochemical Treatment of Municipal Wastewater byMiller and Knipe, a report from the Department of Health, Education, andWelfare [AWTR-13], 1965) that the addition of flocculant willeffectively reduce a small (7-8 ppm.) ABS detergent content ofwastewater, there does not appear to be any indication that otherdissolved carbon constitutents would be removed in this way.

A distinct advantage in the practice of this invention is theeffectiveness with which the dissolved organic carbon content ofwastewater can be substantially reduced. In order to more clearlyillustrate this capability in the experimental runs reported below, rawsewage water was first treated with polyelectrolyte flocculants toprecipitate suspended solids. The flocculated solids were filtered outand the filtrate was used as a feed watsewater for the experimentalruns. The electrolysis cell in each case consisted of electrodesdescribed as follows:

Activated carbon electrode.A porous stainless steel cylinder wasdisposed concentric with a cylindrical nylon screen leaving an annulargap of A; inch. In this annular gap commercially available activatedcarbon granules, passing 12 mesh and being retained on mesh, weretightly packed.

Steel electrode-A carbon steel cylinder was used, the gap between thetwo electrodes being /z inch.

EXAMPLE 1 In the first experimental run the activated carbon electrodewas used as the cathode and the steel electrode was used as the anode. Aconstant volume of the filtrate wastewater was placed in a reservoir andwas repeatedly circulated through the annular space between theelectrodes at a flow rate of 0.7 gallon/min. ftf Commercial oxygen wasfed into the device through the porous cathode cylinder at a flow rateof 0.048 ftfi/min. ft.". The current density employed was 3-6 ma./cm.and the run was continued until 4500 coulombs per liter of water hadbeen applied. Iron oxide in hydrated form, which was produced, wasfiltered out and the dissolved organic carbon content of the filtratebefore and after was determined by means of a Beckman carbon analyzer.It was found that the original dissolved organic carbon in the feed, 89mg./liter, had been reduced to 32 mg./liter. The odor of the sewagewater was also markedly reduced.

EXAMPLE 2 In the second experimental run the polarity was re versedthatis, the carbon electrode was made anode and the steel electrode was madecathode. All the other experimental conditions were the same except foruse of a higher current density, 15 ma./cm. The reduction of dissolvedorganic carbon was much smaller (from 89 mg./l. to mg./l.) and thetreated water in this run still retained the strong odor of the originalwastewater.

Thus, two aspects of this invention have been illustrated by Examples 1and 2; namely, the pronounced reduction in dissolved organic carbon andthe importance in the practice of this invention of making the oxygen(activated carbon) electrode the cathode.

As has been stated hereinabove, the wastewater treated in Examples 1 and2 had suspended solids removed therefrom solely to more clearlyillustrate the effect on dissolved solids, and previously untreatedwastewater can equally well be treated with the apparatus and process ofthis invention, suspended solids being removed by the electrocoagulationeffect, which occurs.

From the experimental Example 1, the reduction of dissolved organiccarbon content is calculated to be 13 mg. of organic carbon/liter by theapplication of 1000 coulombs of electricity/liter. According to theabovementioned Health, Education, and Welfare report straightelectrolysis of the efiluent from a wastewater secondary treatmentprocess reduced the chemical oxygen demand (COD) by 1.7 mg./ 1000coulombs of current. In the particular wastewater employed for theexperiments described above, the organic carbon concentration in mg./l.,was essentially equal to the COD in rug/l. Although an exact comparisonof process efiiciency is rather difficult, because the experimentalconditions employed, including the feed water, differed from those inthe Health, Education, and Welfare study, a significant increase inprocess efiiciency is evident in the electrochemical wastewatertreatment process of this invention as compared to conventionalelectrolysis.

Calculations indicate that the minimum electrical input should be atleast about 33 coulombs to remove one milligram of dissolved organiccarbon by oxidation in the practice of this invention. The minimum flowof oxygen-containing gas should deliver at least about 7 milligrams ofoxygen per liter of wastewater.

What we claim as new and desire to secure by Letters Patent of theUnited States is:

1. Apparatus for the treatment of wastewater primarily for the reductionof organic carbon content comprising in combination:

(a) a wastewater flow compartment having at least one flow channeldefined at least in part by spaced opposed first and second electricallyconducting confining wall areas disposed in electrically insulatedrelationship to each other,

(b) means connected to and in flow communication with said compartmentfor continuously admitting wastewater flow thereto for treatment duringpassage through said flow channel,

(c) means connected to and in flow communication W with said compartmentfor continuously discharging treated wastewater flow therefrom afterpassage thereof through said flow channel, said discharging means beingout of flow communication with said admitting means externally to saidcompartment,

(d) said second confining wall area being porous to the 3. The apparatusof claim 1 wherein the first confining passage of gas therethrough andforming at least part wall area is made of a ferrous metal. of a closedend enclosure having the interior thereof in flow communication with asource of oxygen con- References Cited taining gas, said secondconfining wall area compris- 5 UNITED STATES PATENTS ing an electricallyconductive activated carbon layer catalytic to the electrochemicalformation of hygi ggg imgen Peroxlde 2,000,815- 5/1935 Berl 204.465 (e)electrically conductwe leads in electrical contact 3 076 754 2/1963Evans 2O4 272 with said first and second confining Wall areas, and 10(f) means for applying direct current ower to said leads such that saidfirst and second gonfining wall SOLOMON Asslstant Exammer areas willconstitute anode and cathode electrodes, JOHN MACK, Primary Examinerrespectively. 2. The apparatus of claim 1 wherein the second confin- 15ing wall area is in the shape of a right circular cylinder. 204 4, 149,24 270, 2 4

